Create INSTALL.md

INSTALL.md

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+Install [devkitARM](https://devkitpro.org/wiki/Getting_Started) (if you are on **Windows 10**, [do this instead](#windows-10)).
+
+Run the following commands (first, see [this](#macos) if you are on **macOS** or [this](#old-windows) if you are on **old Windows**):
+
+	export DEVKITPRO=/opt/devkitpro
+	echo "export DEVKITPRO=$DEVKITPRO" >> ~/.bashrc
+	export DEVKITARM=$DEVKITPRO/devkitARM
+	echo "export DEVKITARM=$DEVKITARM" >> ~/.bashrc
+
+	git clone https://github.com/pret/pokefirered
+	git clone https://github.com/pret/agbcc
+
+	cd agbcc
+	./build.sh
+	./install.sh ../pokefirered
+
+	cd ../pokefirered
+
+To build **pokefirered.gba**:
+
+	make -j$(nproc)
+
+If you have only changed `.c` or `.s` files, you can turn off the dependency scanning temporarily. Changes to any other files will be ignored, and the build will either fail or not reflect those changes.
+
+	make -j$(nproc) NODEP=1
+
+
+## macOS
+
+Run `xcode-select --install` in Terminal, then proceed by executing the commands.
+
+
+## Old Windows
+
+*For Windows 8.1 and earlier*
+
+Download and run the [Cygwin](https://www.cygwin.com/install.html) setup, leaving the default settings intact. At "Select Packages", set the view to "Full" and choose to install the following:
+
+- `make`
+- `git`
+- `gcc-core`
+- `gcc-g++`
+- `libpng-devel`
+
+In the Cygwin command prompt, enter the commands.
+
+If the command for building pokefirered.gba does not work, run `nproc` and use that value instead of `$(nproc)` for `make`.
+
+
+## Windows 10
+
+Install the [Windows Subsystem for Linux](https://docs.microsoft.com/windows/wsl/install-win10), then install [devkitARM](https://devkitpro.org/wiki/Getting_Started) inside the subsystem, and run the commands.